1974 450 SEL, 256,000 miles. Down on power, uses a little oil
(but not excessive). Recently a strange problem has developed.
When I start the car cold, everything is normal. No visible smoke.
Then I'll drive about a mile or two. First couple of times, I'd drive to a store, shut it off, come out, start it and there would be an
excessive cloud of blue smoke. Then as I drove on there would be no smoke. Last couple of times, it would start (no smoke), then after 1 or 2 miles at most, if I stopped at a light and then accelerated there would be the excessive amount of blue smoke. Drive on and no visible smoke. When I say excessive, I mean a big cloud, so much that if there were a cop behind me he would probably stop me. But at cold initial startup and when the car is warm there is no visible smoke.
At first I thought there might be a stuck ring. But because there is a regularity to the problem (only does it 1 or 2 miles after startup) I don't think that's it. Also, wouldn't seem to be valve guide seals, because wouldn't that smoke right at startup?
Might there be some emissions related thing that is sucking oil
somehow, or a leaky power brake booster or something?

I'm stymied. Any ideas?